We discuss spectropolarimetric measurements of photospheric (Fe I 630.25 nm) and chromospheric (Ca II 854.21 nm) spectral lines. Our long-term goal is to diagnose properties of the magnetic field near the base of the corona. We compare ground-based t
wo-dimensional spectropolarimetric measurements with (almost) simultaneous space-based slit spectropolarimetry. The ground-based observations were obtained May 20, 2008, with IBIS in spectropolarimetric mode, The space observations were obtained with the Spectro-Polarimeter aboard the HINODE satellite. The agreement between the near-simultaneous co-spatial IBIS and HINODE Stokes-V profiles at 630.25 nm is excellent, with V/I amplitudes compatible with to within 1 %. IBIS QU measurements are affected by residual crosstalk from V, arising from calibration inaccuracies, not from any inherent limitation of imaging spectroscopy. We use a PCA analysis to quantify the detected cross talk. Chromospheric magnetic fields are difficult to constrain by polarization of Ca II lines alone. However, we demonstrate that high cadence, high angular resolution monochromatic images of fibrils in Ca II and H-alpha, can be used to improve the magnetic field constraints, under conditions of high electrical conductivity. Such work is possible only with time series datasets from two-dimensional spectroscopic instruments under conditions of good seeing.
We present novel evidence for a fine structure observed in the net-circular polarization (NCP) of a sunspot penumbra based on spectropolarimetric measurements utilizing the Zeeman sensitive FeI 630.2 nm line. For the first time we detect a filamentar
y organized fine structure of the NCP on spatial scales that are similar to the inhomogeneities found in the penumbral flow field. We also observe an additional property of the visible NCP, a zero-crossing of the NCP in the outer parts of the center-side penumbra, which has not been recognized before. In order to interprete the observations we solve the radiative transfer equations for polarized light in a model penumbra with embedded magnetic flux tubes. We demonstrate that the observed zero-crossing of the NCP can be explained by an increased magnetic field strength inside magnetic flux tubes in the outer penumbra combined with a decreased magnetic field strength in the background field. Our results strongly support the concept of the uncombed penumbra.
